To Wed a Proper Lady: The Bluestocking and the Barbarian — Jude Knight

Everyone knows James needs a bride with impeccable blood lines. He needs Sophia's love more.

James must marry to please his grandfather, the duke, and to win social acceptance for himself and his father's other foreign-born children. But only Lady Sophia Belvoir makes his heart sing, and to win her, he must invite himself to spend Christmas at the home of his father's greatest enemy.
Sophia keeps secret her tendre for James, Lord Elfingham. After all, the whole of Society knows he is pursuing the younger Belvoir sister, not the older one left on the shelf after two failed betrothals.